Response:
Although it's not as common as it once was, there are still people who believe that atheists both believe in and worship Satan, the evil opponent of God. This is an almost literal demonization of atheists, since the primary servants of Satan are always depicted as literal demons. Describing atheists in this manner makes it easy to dismiss them and whatever they say — after all, it would be wrong for a true and faithful follower of God to pay any attention to the lies of Satan's minions.

Christians who repeat this myth are working from a common Christian assumption that, for some reason, only their god is relevant to atheists. So if an atheist does not believe in their god, then they must worship the antithesis of their god, Satan.

The truth is, atheists who don't believe in a god also aren't going to believe in this god's supernatural competitor, either. It's technically true that being an atheist doesn't not exclude belief in anything supernatural at all, only gods. Satan, however, is a specific figure within Christian mythology. Since Christianity is a religion which is focused on belief in and worship of a particular god, atheists aren't going to accept it as their own. It is, therefore, simply not plausible that atheists will believe in Satan.

One scriptural source for this claim may come from Matthew:

No one can serve two masters, for either he will hate the one and love the other, or he will hold to one and despise the other. You cannot serve God and mammon. (Matthew 6:24)

Assuming that the believer interprets "mammon" to include Satan, this verse claims we must either love God and hate Satan or love Satan and hate God. Atheists obviously do not love and serve God, so they must love and serve Satan.

This biblical argument is invalid, however. First, it assumes the absolute truth of the Bible, or at least of that particular verse. This is a circular argument because it assumes something which is at the heart of the disagreement between atheists and Christians. Second, it is an example a False Dilemma fallacy because it assumes that the above are the only two choices. The idea that there may not exist any God or Satan, which would open up a wealth of other possibilities, does not seem to occur anyone offering this.

The closest thing there is to atheist Satan-worshippers are atheists who treat Satan as a sort of metaphorical symbol for particular principles. It is a bit of a stretch to say that they "worship" this principle, though — how does one "worship" an abstract idea? Nevertheless, even if we do allow that it is a form of "worship," their numbers are small and most atheists do not fall into this category. At most we can say that there are some atheists who "worship" a Satan who is not real, but it's not even remotely true that atheists generally or as a class worship Satan — or worship anything at all, for that matter.